Abscisic acid (ABA) is a natural occurring hormone found in all higher plants (Cutler and Krochko 1999, Trends in Plant Science, 4:472-478; Finkelstein and Rock 2002, The Arabidopsis Book, ASPB, Monona, Md., 1-52). ABA is involved in many major processes during plant growth and development including dormancy, germination, bud break, flowering, fruit set, general growth and development, stress tolerance, ripening, maturation, organ abscission and senescence. ABA also plays an important role in plant tolerance to environmental stresses, such as drought, cold, and excessive salinity.
One key role of ABA in regulating physiological responses of plants is to act as a signal of reduced water availability to reduce water loss, inhibit growth and induce adaptive responses. All these functions are related to stomatal closure (Raschke and Hedrich 1985, Planta, 163: 105-118). When stomata close, plants conserve water to survive environmental stresses. However, stomatal closure also can result in reduced photosynthesis, respiration and growth. Stomatal closure is a rapid response of plants to ABA. The mechanism of this effect has been studied and has been shown to be primarily due to ABA effects on guard cell ion channels. Specifically, ABA blocks H+ extrusion and K+ influx from guard cells and promotes K+, Cl−, and malate extrusion and Ca2+ influx. The net effect of ABA is to reduce the total osmotica in the guard cells, which in turn decreases the water content in the cell. This causes the guard cells to lose turgor and thus close the stomata (Assmann 2004, In: Plant Hormones Biosynthesis, Signal Transduction, Action, ed. Davies, p 391-412). The closing of the stomata results in reduced transpiration. The reduction of transpiration caused by stomatal closure is widely used as an experimental technique to indirectly identify and quantify ABA activity. The ability of ABA to reduce water use can not only extend the display shelf life of ornamentals or the postharvest shelf life of leafy plants, or promote drought tolerance, but it also can lead to a reduction in cold stress injury (Aroca et al. 2003, Plant Sci., 165: 671-679). ABA-induced reduction of stomatal conductance can lead to a decrease in photosynthesis (Downton et al. 1988 New Phytol., 108: 263-266) which in turn can lead to growth control. Improving the performance of ABA may be useful not only for improving the reduction of transpiration and water loss, but also for other uses of foliar applied ABA such as maintaining dormancy of buds and seeds, controlling fruit set, accelerating defoliation and enhancing color development of fruit such as grapes.
The exogenous application of ABA is an alternative approach to induce plant response to abiotic stress. However, exogenous ABA entering plant cells can be easily catabolized and, thus the effect of ABA on plants usually lasted a short time.
ABA analogs are chemicals with similar structures as natural ABA. A series of ABA analogs have been developed by the Plant Biology Institute of Canada to mimic ABA function. So far, many ABA analogs have reportedly exhibited ABA-like effects. Compared to natural ABA, ABA analogs are more resistant to degradation. However, the effects of ABA analog treatments varied with the concentration, mode of application (foliar or root-dip) and crop species.
Surfactants or adjuvants have long been used with pesticides and plant growth regulators to increase absorption or uptake by plants and thus improve the performance of the applied chemicals. Adjuvants include wetter-spreaders, stickers, penetrants, compatibility agents and fertilizers. Many adjuvants are currently available and most of them are nonionic surfactants. However, there is little information about adjuvants suitable for ABA analogs. Tween 20, a popular surfactant used in scientific research, was reported to be added as the surfactant for ABA analogs (Waterer, 2000. ADF Project 97000289). However, Tween 20 is used for academic research and not packaged and distributed for the agricultural market.
Foliar applied nitrogen fertilizers, such as urea or ammonium nitrate, have been used in combination with plant growth regulators (PGRs) to improve the performance of the PGR. For example, the combination of the PGRs benzyladenine (Naito et al. 1974, J. Japan. Soc. Hort. Sci., 43: 215-223) or gibberellic acid (Shulman et al. 1987, Plant Growth Regul., 5: 229-234) with urea increased the grape berry sizing effect compared to the sizing effect achieved with the PGR alone. Ammonium salts have been reported to increase the absorption of pesticides (Wang and Liu 2007, Pestic. Biochem., Physiol., 87: 1-8). Nooden (1986, U.S. Pat. No. 4,581,057) claims the use of ABA analogs to increase fertilizer performance. However, there are no reports on the use of urea (H2NCONH2) or ammonium nitrate (NH4NO3) to improve ABA analog performance.
Foliar application of nutrients has been used as an alternative approach to supplement nutrient to crops. Kuepper (2003, Appropriate Technology Transfer for Rural Areas. March 2003) reported that foliar fertilization increased crop yield and quality as well as resistance of crops to biotic and abiotic stress. The major advantage of foliar fertilization over ground fertilization is its efficiency of nutrient absorption. In contrast, root absorption costs energy to transport nutrient from root to shoots. Also, foliar fertilization reduces nutrient loss and ground water contamination.
Foliar applications of nitrogen fertilizer, however, do not always increase the yield of crops as expected. For example, foliar application of urea on soybeans usually decreased the yield (Gray, 1977, Situation 77. Natl. Fertil. Dev. Ctr., Muscle Shoals, Ala. Bull. Y-115). The reduction of soybean yield was thought to be caused by leaf burn due to phytotoxicity of foliar urea fertilization (Krogmeier et al., 1989, Proc. Natl. Acad. Sci. USA. 86:8189-8191). Phytotoxicity of foliar fertilizer was affected by the form of nitrogen fertilizer, concentration of fertilizer and humidity or temperature of application site (Garcia and Hanway, 1976, Agron. J. 68, 653-657; Poole et al., 1983, Agronomy J. 75:201-203). Urea, the most popular nitrogen fertilizer, was often observed to cause leaf burn after foliar fertilization. The application of granular fertilizer also has the potential to cause phytotoxicity leaf burn because leaves of turfgrass are close to the ground and readily contact granular fertilizer.
Bremner (1995, Fertilizer Research 42:321-329) summarized approaches to reduce the phytotoxicity of urea, including (a) addition of an urease inhibitor to fertilizer; (b) coating of the fertilizer with sulfur or other materials to slow its rate of dissolution; (c) acidulation of the fertilizer with inorganic acids; (d) treatment of the fertilizer with inorganic salts; and (e) use of urea supergranules. Some of these approaches can also be used for other nitrogen fertilizers.
Foliar application of calcium supplements is also a typical management practice in many crops and in particular apple and other fruit trees. High doses of calcium also potentially cause phytotoxicity on the tree canopy. However, less attention has been paid to the reduction of calcium phytotoxicity.
Thus there is a need to reduce the phytotoxicity resulting from the foliar application of nitrogen-containing fertilizers or calcium-containing nutrient supplements. A reduction in phytotoxicity would also enable the use of higher fertilizer rates and potentially less frequent applications.
In order maximize the performance of ABA analogs in their various agricultural and horticultural applications; there is a need to improve ABA analog and ABA derivative absorption to reduce water loss and leaf transpiration of plants.